1. Field of the Invention
The present invention relates to a semiconductor single-crystal lift device for producing semiconductor single crystals by using the Czochralski Method (the CZ method).
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-open No. 7-172971, a radiation shield screen is used in the process of lifting crystals for the purpose of accelerating the lift speed, avoiding pollution of impurities and improving the yield of dislocation-free crystals. The radiation shield screen is constructed from an upper screen and a lower screen. The upper screen consists of a three-layer structure, which has two outer graphite members and an inner adiabatic material made of graphite or ceramic fibers disposed therebetween. The lower screen comprises a one-layer structure and is made of graphite, quartz or fine ceramics.
In a semiconductor single-crystal lift device provided with above-described radiation shield screen, the radiation shield screen is used to shield single crystals from the radiation coming from the melt and crucible made of quartz, and to expedite the cooling of single crystals. In other words, the radiation shield screen is used for heat obstruction. By way of omitting the adiabatic member disposed near the free surface of the melt, the high temperature zone (above the free surface of the melt) of over 1200.degree. C. can be expanded so as to prolong the time required for passing grown crystals through the high temperature zone. To obtain high dielectric strength of oxidation layers formed on semiconductor wafers, it is necessary to prolong to the maximum the time single crystals pass through the high temperature region (at 1000.degree. C. to 1200.degree. C. ). The above requirement is nowadays well-known to those skilled in this art.
However, in the above mentioned prior art process, at temperatures ranging from 1000.degree. C. to 1200.degree. C., single crystals cool quickly, which causes a decline in dielectric strength of oxidation layers formed on single crystals. On the other hand, if the lifting speed is reduced, dielectric strength of oxidation layers formed on single crystals is enhanced, yet the yield of single crystals decreases. In conclusion, to enhance dielectric strength of oxidation layers formed on single crystals, it is essential to reduce the cooling speed of single crystals at temperatures ranging from 1000.degree. C. to 1200.degree. C. Furthermore, to reduce the amount of OSF(Oxygen Stacking Fault) and oxygen precipitation, it is required to accelerate the cooling speed of single crystals at temperatures below 1000.degree. C.